By default, Apple Time Machine does not back up:

• Cache files
• Files in the Trash
• Files in the hidden directory/tmp/

By default, Apple Time Machine does not back up:

• System log files
• Spotlight indexes
• Cache files of any type
• Files in the Trash (or any other user's Trash)
• Temporary files
• Specific application data that an app may flag for Time Machine to not back up

This applies to all versions of Mac OS X which have Time Machine.

Source: Kevin M. White.Apple Training Series: Mac OS X Support Essentials v10.6. Peachpit Press.